HYPERTHERMIA CAN REDUCE CYTOTOXICITY FROM ETOPOSIDE WITHOUT A CORRESPONDING REDUCTION IN THE NUMBER OF TOPOISOMERASE-II DNA CLEAVABLE COMPLEXES

The chemotherapeutic drug etoposide (VP-16) causes the equilibrium rea ction between noncleavable and cleavable topoisomerase II-DNA complexe s to shift in favor of the cleavable complex [H. Zang, P. D'Arpa, and L. F. Liu, Cancer Cells (Cold Spring Harbor), 2: 23-27, 1990]. Pulsed- field gel electrophoresis was used to study induction and removal of c leavable complexes in cells heated before, during, or after VP-16 trea tment. Pulsed-field gel electrophoresis results were evaluated both as the fraction of activity (DNA) released from the plug and as the numb er of double-strand breaks (DSBs) calculated from molecular weight dis tributions; both end points led to the same conclusions. When cells we re heated at 42 degrees C during treatment with VP-16 (12 mu g/ml up t o 60 min), a slight decrease in cleavable complexes (from 30 to 20 DSB s/100 megabase pairs) was detected immediately after treatment when co mpared with cells treated with the drug at 37 degrees C. Furthermore, heating at 42 degrees C caused a slight decrease in drug cytotoxicity as measured by less than a 2-fold increase in clonogenic survival. Whe n cells were heated for 10 min at 45.5 degrees C prior to or after tre atment with the drug, there was a reduction (similar to 50%) immediate ly after treatment in the number of DSBs/100 megabase pairs compared w ith unheated cells. The rate of removal of cleavable complexes was dec reased slightly by heat. After 120 min at 37 degrees C, the number of DSBs/100 megabase pairs decreased to similar to 6 for both unheated ce lls and those heated prior to drug treatment and to similar to 8 for c ells heated after drug treatment. In agreement with a low effect of he at on the number of cleavable complexes after drug treatment, there wa s no significant effect of this heating protocol on drug cytotoxicity. However, heating at 45.5 degrees C prior to drug treatment at 37 degr ees C protected cells from drug cytotoxicity (e.g., increased survival after 12 mu g/ml for 60 min by similar to 100-fold) despite the simil arity in the induction and rate of removal of cleavable complexes when compared with nonheated cells. Thus, when cells are heated prior to a dministration of VP-16, drug cytotoxicity does not correlate with the number of cleavable complexes measured either immediately after treatm ent or 180 min later when similar to 75% of the initial number have be en removed. Finally, since hyperthermia can actually decrease drug cyt otoxicity, the use of hyperthermia as an adjuvant to chemotherapy invo lving topoisomerase II poisons, such as VP-16, should be approached wi th caution.